1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member provided with a photosensitive layer containing at least amorphous silicon (hereinafter referred to as a-Si) on a cylindrical conductive substrate, as well as to an image forming apparatus provided with the electro-photographic sensitive member, such as a copying machine, a printer, that forms images utilizing the electrophotographic process.
2. Related Background Art
A conventional image forming apparatus, such as a copying machine, a printer, a facsimile apparatus, utilizing the electrophotographic process, is arranged to form images by a charging process to charge the electrophotographic photosensitive drum (hereinafter referred to as a photosensitive drum) evenly and uniformly, which serves as an image bearing member in general; by a latent image forming process to write electrostatic latent images on the photosensitive drum; by a developing process to develop the electrostatic latent image with toner (developer); by a transfer process to transfer toner on the photosensitive drum to a transfer material; and by a fixing process to fix the toner on the transfer material.
Then, after the transfer process, the residual toner which remains on the photosensitive drum is removed by use of a cleaning device.
As the cleaning device that removes the residual toner remaining on the photosensitive drum after transfer, various ones have been proposed conventionally. For example, there is the cleaning device which scrapes off the residual toner remaining on the photosensitive drum after transfer by the cleaning blade formed by urethane rubber or some other elastic material, and which is widely in use because this device presents an excellent function of toner removal, while it can be structured compactly at lower costs.
However, it is conceivable that the particles, which adhere to the surface of the photosensitive drum to affect the image quality, are not only the toner, but also, the fine paper particles created from paper used for most of the transfer materials, the organic component extracted from the fine paper particles, and the corona product created due to the presence of the high-voltage member in the apparatus, among some others. If these particles as foreign substances should adhere to the surface of the photosensitive drum, it tends to impede the formation of clear electrostatic latent images, because these foreign substances present lower resistance condition under high humidity environment, thus inviting the degradation of image quality eventually.
It has been known that the a-Si photosensitive drum that forms film by the glow discharge decomposition of silane is easier to be affected by the above-mentioned phenomenon that may lead to the image degradation. In order to prevent the creation of such phenomenon of the image degradation, a method has been proposed to rub and remove the various foreign substances described above by the grinding function of toner particles in the location of the cleaning blade. The blade is spaced in such a manner that when the magnetic toner of monocomponent type is used as the developer in particular, the magnetic roller is arranged on the upstream side of the cleaning blade in a running direction (rotational direction) of the photosensitive drum so as to form the magnetic brush by use of a part of the toner (residual toner after transfer) collected by the cleaning blade. The magnetic brush thus formed is in contact with the surface of the photosensitive drum to supply the magnetic toner again.
As compared with the method involving rubbing the surface of the photosensitive drum by grinding element, such as a web, a rubber roller, which is separately prepared, the above-mentioned method has a lesser degree of the grinding function biased toward the surface of the photosensitive drum. Then, it may be possible to apply this method to the so-called Carlson photosensitive drum which enables charges to be transported in the photosensitive layer of the photosensitive drum at the time of exposure.
Therefore, this method produces a specific effect on the prevention of the image degradation resulting from the causes described above when it is used in combination with a method in which the surface of the photosensitive drum is prevented from presenting the lower resistance with the arrangement of a heater for the photosensitive drum, for example, so as to make humidity lower on the circumference of the drum at night or during standby.
In recent years, to address problems related to the environment or energy saving there is a growing demand that the power dissipation is reduced to a minimum at night or during standby. Also, under high humidity, the heater provided for the a-Si photosensitive drum should be energized. Otherwise, the foreign substances, such as corona product, which may adhere to the surface of the photosensitive drum, tend to promote a lower resistance on the surface of the photosensitive drum and invite the degradation of the image quality, such as running of images. For the reasons given below, it is conceivable that the intended rubbing removal of the lower resistive substance, such as discharge product, together with toner from the surface of the photosensitive drum is not completed exactly even by use of the cleaning device.
(1) The rubbing member in the cleaning device mainly rubs the photosensitive drum with the residual toner after transfer which exists between the rubbing member and the photosensitive drum. However, the residual toner after transfer may, in some cases, contain talc (whose chemical composition is: Mg.sub.3 (Si.sub.4 O.sub.10)(OH).sub.2) as a filler of the transfer material and other hygroscopic materials. As a result, it is conceivable that under high humidity, the surface resistance of the photosensitive drum is locally lowered to 10.sup.11.OMEGA. approximately to make it easier to create the running of images.
(2) There are some cases where the ratio of the external additives contained in toner becomes higher in the residual toner after transfer. Particularly when the external adhesives provide the polarity which is opposite to the charged polarity of the toner, this tendency is more conspicuous. Then, in this case, the ratio of the external additives becomes higher in the cleaning location to make the rubbing force smaller locally for the photosensitive drum. Conceivably, therefore, this phenomenon causes the resultant degradation of image quality.
(3) With the arrangement of the magnet roller on the upstream side of the cleaning blade for the formation of the magnetic brush with a part of the toner collected to the cleaning device, the magnetic toner is supplied again with such magnetic brush which is in contact with the surface of the photosensitive drum, then various foreign substances which have adhered to the surface of the photosensitive drum are removed by rubbing by the application of grinding forces using the toner particles in the blade position. In this case, it is conceivable that the grinding force by the toner particles is not effectuated uniformly in the longitudinal direction of the photosensitive drum.
Particularly, on both ends of the cleaning blade, the contact pressure of the cleaning blade becomes higher inevitably. Further, on both ends of the cleaning blade, toner is not supplied to the magnet roller sufficiently. The resultant coating amount of toner becomes smaller, and in some cases, the toner is not supplied to the cleaning blade sufficiently. Under such conditions, the cleaning blade vibrates on both ends thereof in particular, making it impossible to remove toner on the surface of the photosensitive drum completely which results in defective cleaning.
Here, not only the effect of grinding force on both ends of the cleaning blade is smaller, but also the surface layer of the photosensitive drum is ground unevenly by the cleaning blade in some cases.